Geophysical prospecting



Nov. 6, 1962 D. COURSEN ETAL 3,062,313

GEOPHYSICAL PROSPECTING Filed Nov. 12, 1957 Iii (5.1

4a 4b 4c 8 4d 4e 4f 4 \VMNMQ/ /AQ A I l INVENTORS DAVID L. COURSEN NORMAN G. JOHNSON ATTORNEY United States Patent Ofitice aaazsia Patented Nov. 6, 1962 3,062,313 GEOPHYSICAL PROSPECTING David L. Coursen, Newark, and Norman G. Johnson, Wilmington, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Nov. 12, 1957, Ser. No. 695,804 2 Claims. ((Jl. 181-.5)

The present invention relates to an improved assembly and method for conducting geophysical surveying. More particularly, this invention relates to an improved method and assembly for producing and recording the sonic pulse required for geophysical surveying. This application is a continuation-in-part of our copending application Serial Number 618,509, filed October 26, 1956, now abandoned.

Geophysical exploration techniques generally involve the study of reflected or refracted transient sonic pulses which are generated near or at the surface. The principal source for the sonic pulse is the detonation of an explosive charge or a plurality of charges. These charges may be positioned on the surface, above the surface, or under the surface. The use of charges on the surface is employed to the largest extent when the strata under a body of water are being explored. Some recent work on the use of a number of charges arranged in a pattern and elevated 'a few feet above the surface has produced excellent records. However, because the weathered layer of earth at the surface is a relatively poor transmitter of the pulse from an explosion, the greatest proportion of all geophysical surveying involves the positioning of the charge from 25 to 400 feet under the surface of the earth, i.e., in a borehole extending through the weathered layer of earth to higher density strata. For optimum transmission of the pulse from the detonation of the charge, the borehole is usually filled with water.

Regardless of the positioning of the explosive charge, the only completely successful means for detonating the charge or charges has been by means of electric blasting initiators positioned in initiating relationship to the charge or charges. The interpretation of the signal recorded by the geophones in an array on the surface is directly dependent upon a precise knowledge of the time at which the charge or charges detonated. By the use of electric initiators having a minimum (less than 0.5 millisecond) interval between actuation and the application of a high (5 amperes or more) firing current, the time of application of the firing current provides the needed record. Another method employs a low (1 ampere or less) firing current in which the explosion of the cap breaks the electrical circuit and provides the timebreak. From the standpoint of accuracy and dependability, electric initiators have served the needs of the surveying industry very well.

In the early days of search for petroleum, most of the areas under survey were remote from populated or industrialized sites. Because of the tremendous need for continued discovery of new sources of petroleum, geophysical prospecting is being carried out at an ever increasing pace in both remote and settled areas. This increasing pace and approach to settled areas, the latter in many cases being due to the expansion of population and industry into formerly remote areas, has brought on many new problems and emphasized old problems.

In settled areas, the noise produced by the detonation of explosive charges on or above the surface is objectionable, and, therefore, only underground shooting can be tolerated. In such areas, electrification has occurred, and many powerlines traverse the area. Many non-fatal and several fatal accidents have resulted from lead wires thrown out of a borehole and across such powerlines. In

view of the relatively high cost of drilling boreholes, and the feasibility of reusing a hole for a number of shots for record purposes, the prevention of thrown wires by means of stemming is not practicable. Various mechanical wire-catchers have been designed and used, but all such devices present transportation and set-up difiiculties, and their effectiveness is not as high as desired. The substitution of Primacord detonating fuse for the lead wires is in use to a limited extent. However, when the detonating fuse is brought entirely to the surface of the ground, the disturbance caused by its detonation is so severe that a satisfactory record of the shot cannot be obtained. A modified scheme in which the electric initiator is used to initiate detonating fuse 10 to 15 feet down the borehole overcomes partially the disturbing effect of the detonation of the fuse and, at the same time, shortens the lead Wires so that they cannot reach from a power line to the ground. Even this scheme requires such troublesome procedures as desensitizing the uphole geophone and moving it 15 to 20 feet away from the borehole in order to obtain the required correction for the weathered layer. Furthermore, the disturbance produced by Primacord may interfere with the first return of refracted seismic energy.

Another problem associated with the use of electric initiators is the likelihood of premature initiation of the charge. In practically any operation, the charge must be positioned quite some time before the shot is made. Obviously, the initiator must be installed with the charge. The charge then becomes susceptible to accidental ignition by stray electrical currents, electrical storms, and, as cases on record show, by unauthorized persons. The problem is particularly acute when the charges are positioned several days before a shot is made. In many cases, such preloading represents the only practicable procedure because several drilling crews are required to keep up with the recording crew and the drilling crews must load the charges because the boreholes may not stay open until the arrival of the recording crew.

Accordingly, an object of this invention is to provide an assembly for geophysical prospecting by means of an array of geophones on the surface and the production of seismic waves by detonation of an explosive charge wherein the charge is initiated by a non-electrical initiation means which does not interfere with the shot records. A further object is to provide an assembly for producing and recording sonic pulses characterized by simplicity and safety. A still further object is to provide a method for producing and recording a sonic pulse wherein only nonelectric initiating means are connected to the explosive charges. Additional objects will become apparent as this invention is more fully described.

The foregoing objects are achieved by providing an arrangement comprising an array of geophones on the surface and an explosive charge positioned at a predetermined shot point, a non-electric initiator in initiating relationship to the explosive charge, and a length of an explosive connecting cord comprising a core of from 0.1 to 5.0 grains per foot of length of a high velocity detonating explosive within a metal sheath, wherein said length of explosive connecting cord extends from said initiator to a convenient location for initiation thereof.

The connecting cord, described and claimed in application Serial No. 597,145, filed by D. J. Andrew, R. W. Felch, and G. A. Noddin on July 11, 1956, now abandoned, has very little brisance and produces very much less noise than detonating fuse. Therefore, the disturbance effect which renders detonating fuse unsatisfactory is minimized to such extent as to no longer present a problem. On the other hand, the connecting cord propagates detonation at substantially the same velocity as detonating fuse and possesses the same uniformity in the propagation velocity. The explosive core consists of the type of composition commonly used in detonating fuses, i.e., PETN pentaerythritol tetranitrate), RDX (cyclotrimethylenetrinitramine), nitromannite, lead azide, TNT (trinitrotoluene), HMX (cyclotetramethylenetetranitramine), lead styphnate, tetryl, etc., the foregoing all being classified as high explosives. As the metal sheath, any of the ductile metals and alloys may be used, lead and tin being preferred. The metal sheath will generally be reinforced with a fabric countering or with a coating of a flexible plastic material such as polyethylene, polyvinyl chloride, etc. to provide additional strength and also to increase the outside diameter of the connecting cord to permit its use with conventional blasting supplies. The cord can be initiated by a conventional detonator.

As the non-electric initiator, any of the conventional ordinary blasting caps may be used. These caps comprise a metal shell containing a base charge of a detonating explosive, for example, PETN, RDX or tetryl, a priming charge, for example, lead styphnate. The end of the connecting cord is inserted into the open end of the metal shell and the shell fastened to the end of the cord, for example, by peripheral crimps. Such an initiator is needed inasmuch as the connecting cord does not have suflicient brisance to initiate the explosive composition normally used for seismic prospecting.

The length of the connecting cord may extend from the explosive charge to any point at which an electric initiator can be conveniently attached. For example, when the explosive charge is located in a borehole near power lines, and the main concern is to avoid throwing a conductor over the lines, the connecting cord need be of sufficient length just to shorten the lead Wires to the point that they cannot reach the power lines. The metal sheath in the connecting cord is disintegrated by the detonation of the explosive core, therefore, no unwanted electrical circuit can be produced by it. On the other hand, because the cord does not cause sufiicient disturbance to spoil the record obtained by the geophone array on the surface, it may be desirable to extend it beyond the top of the hole a substantial distance. Such extension is particularly desirable if several shots are to be made simultaneously or in sequence. As previously mentioned, the connecting cord propagates detonation at a uniform rate. If equal lengths of cord are used to initiate the charges, and all of the cords are initiated by a single source, simultaneous detonations of all charges is insured. Similarly, if sequence firing is wanted, the lengths of cord can be adjusted to provide the desired sequence.

To more fully describe the present invention, reference is now made to the accompanying drawing in which FIG- URE 1 represents one embodiment of this invention, and FIGURE 2 represents a detailed view of the connecting cord assembly used in the embodiment of FIGURE 1.

Referring now to FIGURE 1, 1 represents a borehole filled with water 2. At the bottom of the borehole 1 is the explosive 3 and on the surface is seismograph 4 connected to geophones 4a, 4b, 4c, 4d, 4e, 4 and 4g by means of cable 16. Unit 4d is the uphole geophone.

In initiating relationship to explosive 3 is a non-electric initiator 5. Connected to initiator 5 and extending to the top of borehole 1 is the length of connecting cord 6.

Fastened to connecting cord 6 is an electric initiator 7 having leg wires 8.

As shown in FIGURE 2, the initiator 5 consists of a metal shell 5a, base charge 9, priming charge 10, and ignition charge 11. The; connecting cord 6 consists of an outside covering 12, a metal sheath 13 and an explosive core 14. Initiator 7 is connected to cord 6 by means of tape 15.

The connecting cord having from 0.1 to 5.0 grains of high explosive per foot of length encased in a metal sheath is critical to this invention. An explosive core of greater weight per unit of length will create too much disturbance to the geophones for satisfactory use. When less than 0.1 grain per foot is present, the explosive core may fail to propagate detonation reliably.

The invention has been described in detail in the foregoing. We intend, therefore, to be limited only by the following claims.

We claim:

1. An assembly for geophysical prospecting comprising an explosive charge positioned in a borehole at a predetermined shot point, an initiator positioned in initiating relationship to said explosive charge, said initiator being adapted for detonaton by an explosive cord,'a length of explosive connecting cord connected to and extending from said initiator to a firing means at a point remote from said explosive charge, said cord consisting of a core of from 0.1 to 5.0 grains of high-Velocity detonating'explosive per foot of length contained by a metal sheath, an array of geophones positioned in detecting relationship to said explosive charge and' a seismograph connectedto each of said geophones.

2. The assembly of claim 1 wherein saidinitiatorcomprises a metal shell integrally closed at one extremity housing in sequence from said closed extremity a. base charge of a detonating explosive composition, a priming charge of a heat sensitive explosive composition and an, ignition charge.

References Cited in the file of this patent,

UNITED STATES PATENTS 468,890 McFarland Feb. 16, 1892 882,154 Lheure Mar. 17, 1908 2,133,484 Sherar Oct. 18, 1938 2,239,123 Stoneking Apr. 22, 1941 2,309,817 Athy Feb. 2, 1943 2,545,380 Poulter Mar. 13,1951 2,556,299 Scott June 12, 1951, 2,675,882 Bazzoni et a1 Apr. 20, 1954 2,706,011 Bayhi Apr. 12, 19.55 2,775,200 Guenter Dec. 25, 19,56.

FOREIGN PATENTS 493,862 Canada June 23,1953? 742,300 Great Britain Dec. 21, 19,551 747,878 Great Britain Apr. 18, 1956,

OTHER REFERENCES Jakosky: Exploration Geophysics, publishedf by-Tija' Publishing Co., Newport Beach, Calif., 1950, page 862. 

